1. Field of the Invention
The present invention relates to capacitance measurement, and more particularly, to an apparatus and method for measuring changes in capacitance.
2. Description of the Related Art
The acceleration or position of an electrostatic accelerator or an actuator can be measured be detecting changes in capacitance of the electrostatic accelerator or the actuator. However, an offset is produced by parasitic capacitance induced in the electrostatic accelerator or the actuator, and changes in capacitance cannot be precisely detected because of this offset.
A number of techniques have been developed to compensate for an offset produced by the parasitic capacitance. One technique is disclosed in an article by Anthony Hugh Falkner, xe2x80x9cThe Use of Capacitance in the Measurement of Angular and Linear Displacementxe2x80x9d, IEEE Transactions on Instrumentation and Measurement, Vol. 43, No. 6, pp. 939-942, December, 1994. The conventional technique disclosed in the article is to detect changes in capacitance using an asymmetric square-wave modulating signal. However, this conventional technique has a problem in that the gain of a detector will vary due to parasitic capacitance induced around capacitors C1, C2, and C3 shown in FIG. 7 of the article.
Another technique is disclosed in an article presented by Scott A. Valoff and William J. Kaiser, xe2x80x9cSelf-Balancing Interface Circuit for Presettable Micromachined MEMS Accelerometersxe2x80x9d, IEEE Transducers 99, pp. 814-817, June 1999. The conventional technique disclosed in the article is to compensate inconsistency in capacitance in presettable MEMS accelerometers using an asymmetric square-wave modulating signal. However, the conventional technique has a drawback in that the gain of a detector varies though distortion in a detected signal. Consequently, the above two techniques have a problem in that changes in capacitance cannot be precisely detected.
To solve the above problems, it is a first object of the present invention to provide an apparatus for measuring a change in capacitance that can compensate for an offset produced by parasitic capacitance while not affecting the detected change in capacitance.
It is a second object of the present invention to provide a method for measuring a change in capacitance performed by the apparatus for measuring a change in capacitance.
To achieve the first object, the present invention provides an apparatus for measuring a change in capacitance in an object modeled as first and second capacitors connected in series between first and second terminals, wherein the capacitance of one of the first and second capacitors increases by a certain amount and that of the other decreases by the same amount when a perturbation is applied. The apparatus includes: a first gain adjuster that adjusts the gain of an input signal and outputs the gain-adjusted input signal to the first terminal as a first modulating signal; a second gain adjuster that adjusts the gain of an inverted input signal and outputs the gain-adjusted inverted input signal to the second terminal as a second modulating signal; an amplifier that amplifies a modulated signal output from a junction between the first and second capacitors and outputs the amplified result; a demodulator that demodulates the amplified result received from the amplifier in response to a control signal and outputs the demodulated result; a controller that generates the control signal per unit period of the input signal and outputs the generated control signal to the demodulator; and a change measuring unit that measures the change in capacitance from the demodulated result received from the demodulator.
To achieve the second object, the present invention provides a method for measuring changes in capacitance including the steps of: (a) adjusting the gain of an input signal to obtain a first modulating signal and adjusting the gain of an inverted input signal to obtain a second modulating signal; (b) amplifying a modulated signal measured at a junction between the first and second capacitors; (c) demodulating the amplified result in response to a control signal; and (d) measuring the change in capacitance using the demodulated result.